The formation of asexual spores (conidia) by Aspergillus nidulans is a developmental process which can be induced only after a spore-originated colony has reached a certain age (the time of competence). Once competence is acquired, an inducing stimulus results in the appearance of new conidia after a fixed maturation period. Thermo-sensitive aconidial mutants are being used to define the steps in the conidiation process. Most mutants are blocked in steps that occur after induction and are thus maturation functions. One mutant fails to become competent. This competence mutant has a complex phenotype: it greatly overproduces one particular protein and several low molecular weight compounds. Genetic and biochemical studies on this mutant are aimed at understanding its aberrant regulation. The enzyme p-diphenol oxidase (laccase) is responsible for the dark-green conidial pigment and is developmentally regulated. Recessive laccase-negative mutants produce yellow conidia. We are seeking to identify regulator genes for this enzyme via mutants which are either dominant-yellow or constitutive for laccase expression. Such mutants have been isolated and are under study. Our finding that a second laccase is associated with the developing ascocarp (cleistothecium) provides a biochemical marker by which to follow sexual development in a quantitative way. In addition, the kinetics of appearance and the histological location of the enzyme suggest that it plays an important role in cleistothecial morphogenesis. Morphogenic studies are being pursued by means of acleistothecial mutants some of which are deficient in the associated laccase.